1. Field of the Invention
The present invention relates to a process for producing L-ascorbic acid, so called vitamin C.
2. Description of Related Art
L-Ascorbic acid has been industrially produced by the process reported by Reichstein and Grussner (Helv. Chim. Acta., 17, 311-328 (1934)) or improved processes thereof. In these processes, 2-keto-L-gulonic acid is synthesized from D-glucose through many reaction steps, esterified, and reacted with sodium methoxide in methanol to give the corresponding lactone, which is then acidified with hydrogen chloride gas to give L-ascorbic acid. Recently, fermentation processes for producing 2-keto-L-gulonic acid from L-sorbose were found (e.g., JP-A 60-70073), and it has been demanded to establish more advantageous processes for producing L-ascorbic acid from 2-keto-L-gulonic acid.
For example, the following processes for producing L-ascorbic acid from 2-keto-L-gulonic acid have been known: (1) a process comprising reacting 2-keto-L-gulonic acid with conc. hydrochloric acid in an organic solvent (U.S. Pat. No. 2,462,251), (2) a process comprising reacting 2-keto-L-gulonic acid with a mineral acid in the presence of a surfactant in an inert solvent (JP-B 48-15931), (3) a process comprising reacting 2-keto-L-gulonic acid with anhydrous hydrochloric acid gas in an inert solvent in the presence of a surfactant (JP-A 63-500454), etc. However, the above processes do not necessarily provide satisfactory yields of L-ascorbic acid in industrial production. In addition, because the above processes produce a large amount of impurities as by-products responsible for coloration, purification procedures are complicated. Thus, the above processes are not industrially practical.
JP-A 64-79165 discloses a process for producing L-ascorbic acid which comprises reacting 2-keto-L-gulonic acid with an aqueous solution of conc. hydrochloric acid combined with hydrogen chloride gas in the presence of a surfactant in an inert solvent using an aliphatic ketone as a coexisting solvent. This process has achieved a high yield of the desired product by optimizing the amounts of water and hydrochloric acid independently. The presence of aliphatic ketones, which is one of the features of this process, is essential to maintain a high concentration of hydrochloric acid in the reaction system. Aliphatic ketones are, however, decomposed by acid catalysts, and this is a drawback of this process. For example, when acetone is used as an aliphatic ketone, undesirable reactions of acetone such as aldol reaction are inevitable, and 4-chloro-4-methyl-2-pentanone, mesithyl oxide, 4-hydroxy-4-methyl-2-pentanone, etc., are produced. Therefore the recovery ratio of acetone is not necessarily high. In addition, degradation products of ketones cause coloration and a bad smell. It is thus difficult to establish industrial process for producing L-ascorbic acid using a ketone as a coexisting solvent.